Environment sets the environment position when copying to maintain the use lists.

runtime/vm/intermediate_language.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/intermediate_language.h"
 
 #include "vm/bit_vector.h"
 #include "vm/dart_entry.h"
 #include "vm/flow_graph_allocator.h"
 #include "vm/flow_graph_builder.h"
@@ skipping 1512 matching lines @@
                          intptr_t fixed_parameter_count)
     : values_(definitions.length()),
       locations_(NULL),
       fixed_parameter_count_(fixed_parameter_count) {
   for (intptr_t i = 0; i < definitions.length(); ++i) {
     values_.Add(UseDefinition(definitions[i]));
   }
 }
 
 
-Environment* Environment::Copy() const {
+// Copies the environment and updates the environment use lists.
+void Environment::CopyTo(Instruction* instr) const {
   Environment* copy = new Environment(values().length(),
                                       fixed_parameter_count());
   GrowableArray<Value*>* values_copy = copy->values_ptr();
   for (intptr_t i = 0; i < values().length(); ++i) {
-    values_copy->Add(values()[i]->CopyValue());
+    Value* value = values()[i]->CopyValue();
+    values_copy->Add(value);
+    UseVal* use = value->AsUse();
+    if (use != NULL) {
+      use->set_instruction(instr);
+      use->set_use_index(i);
+      use->AddToEnvUseList();
+    }
   }
-  return copy;
+  instr->set_env(copy);
 }
 
 
 #undef __
 
 }  // namespace dart